1. Field of the Invention
The present invention relates to a secondary battery, and more particularly to a secondary battery having increased capacity.
2. Description of the Related Art
In general, a secondary battery (for example, a lithium ion battery) may be classified into a square-type battery, a cylindrical battery, or a pouch battery. The square-type battery includes a hexahedronal can having an open side, an electrode assembly inserted into the can and adapted to be charged/discharged at a predetermined voltage, an electrolyte injected into the can to enable ions to move, and a cap assembly for covering the top of the can to prevent the electrode assembly and the electrolyte from escaping to the exterior. The cap assembly includes a cap plate coupled to the open region of the can with a negative electrode terminal coupled to the center thereof via an insulation gasket.
The electrode assembly is formed by laminating a negative electrode plate, a separator, and a positive electrode plate and winding them a number of times into a jelly roll configuration. A first end of a positive electrode tab is connected to the positive electrode plate and a second end is welded to the cap plate of the cap assembly.
Similarly, a first end of a negative electrode tab is connected to the negative electrode plate and a second end is connected to the negative electrode terminal of the cap assembly. Therefore, the cap plate and the can act as a positive electrode except for the negative electrode terminal formed on the cap assembly.
Such a conventional secondary battery has a problem in that the positive and negative electrode tabs welded to the electrode assembly have a relatively large thickness, and therefore the number of times the electrode assembly can be wound is limited. Particularly, the number of times the electrode assembly can be wound is reduced in proportion to the thickness of the positive and negative electrode tabs. As a result, the capacity of the secondary battery decreases. In addition, the positive and negative electrode tabs have insulation tape attached to a surface thereof to prevent them from being short-circuited to the negative and positive electrode plates having the opposite polarity, respectively. This further reduces the number of times the electrode assembly can be wound, as well as reducing the capacity of the secondary battery.
During the manufacturing process of such a conventional secondary battery, the positive and negative electrode tabs must be separately welded to the positive and negative electrode plates of the electrode assembly, respectively, as well as to the cap plate and the negative electrode terminal of the cap assembly. This makes the manufacturing process complicated and decreases the production yield.
The electrode assembly contained in the can may have a volume similar to the internal volume of the can during assembly and may float inside the can even when its volume is slightly smaller than the internal volume of the can. Such floating causes the positive electrode plate formed on the outermost surface of the electrode assembly to intermittently contact the can. The output voltage or current of the secondary battery may then fluctuate and the positive and negative electrode tabs may even separate from the electrode assembly or the cap assembly. Noise may also be generated as the electrode assembly floats inside the can.